Microphone arrangement



April 1957 H. GROSSKOPF ETAL 2,787,671

' MICROPHONE ARRANGEMENT Filed Sept. 29, 1955 I N VEN TORS United States Patent-O MICROPHONE ARRANGEMENT Herbert Grosskopf, Hamburg, and-Hans Diestel, Hamburg-'Wandsbek, Germany, assignors' to Schall-Techmk Dr. Ing. Karl Schoeps, Karlsruhe-Durlach, Germany Application September 29, 1953, Serial No. 383,028 Claims priority, application Germany October '6, 1952 7 Claims. (Cl. 179-111) The present invention relates to electro-acoustical transducers, and in particular to that class of transducers known as microphones.

Directional microphones, and in particular condenser type microphones are well known. Withresp'ect to single microphone arrangements, the directional e'fi'e'ct 'is obtained by exciting the microphone membrane, or diaphragm, at the rear face thereof by sound waves which first pass over a phase displacing member, while the microphone membrane is directly excited by the same sound waves at the front face thereof. To obtain a onesided directional pattern the phase displacing member is so dimensioned as to effect a time delay of the sound Waves approaching the microphone from a direction behind the same which is equal to the time it takesfor the same sound waves to strike the front face of the microphone membrane. As a result when sound waves are propagating toward the microphone from a direction behind the same, due to the effect of the phase displacing member, the pressure exerted by the sound waves on the front and rear faces of the microphone membrane,

respectively, will be the same. Hence, no net'force will be exerted on the membrane. In'other words the pres sure gradient on the front and rear faces of the microphone membrane will be zero. I

On the other hand, for sound waves coming from a point in front of the microphone, thesoundfstrikiri-g the rear membrane, as compared to "the sou id striking'the front membrane, will have an i'ncrease'd'path length. The

increased path length results'from addeddistanc'e of sound travel due to the sound path deviation and the time delay effected by the phasedisplacing'member. -As

a result, for sound coming from'a point'i n frontofthe microphone the pressure. producedon the "front "and rear faces of the microphone membrane arexno longer in phase. Hence, a netforce will"b"e"experienced"on the membrane which results in an output voltage"a'cross'the output terminals of the microphone. in condenser type microphonesg'the' phase displacement is obtained by means of a phase displacing"member'con- .sisting of an acoustical resi'stanceand'an'acoustical"capacitance, the phase displacing member being'in direct contact with the microphone "membrane.

'A very narrow rear'w'ar'dly exteridingslitmayservees an acoustical resistance. This "slit should preferably be in direct communication with'lthesurromidirig atmosphere as well as with an air chamber'normally"providedbehind the membrane of a microphone. The"rnicroplrone 'arrangement usingthe narrow slip-althsugh ifyie'lds a frequency proportional phase displacementfh'as"not been favored by those skilledin the artsincetheuseof such rearwardly extending slit is notvery jpractical,predominant'iy because "of the many difficulties "experienced in the manufacture of such "a"microphone.

"In order to avoid the aforementioned'fdifficulties ithas been proposed to provide the rear electrode jpartlywith perforations andpartly"with'irecesses. .'Tn'this"case the sound entersthrough"therearpertion cfihe'microphom Patented Apr. 2, 1 957 via the perforations in the rear electrode. These perforations in turn communicate with an inter-electrode air film into which the sound waves pass and from the latter into the recesses provided in the rear electrode. In this arrangement the acoustical resistance is provided by a number of individual air gaps, or slits, formed between the microphone membrane and the rear electrode and defined between adjacent edges of the perforated and recessed sections, in other words over the uninterrupted surface portions of the rear electrode, while the acoustical capacitance is provided by the recessed sections of the rear electrode. It is apparent that with such an arrangement, since neither'the acoustical resistance nor the acoustical capacitance is concentrated but is instead distributed over the entire surface of the rear electrode at points of varying membrane slack resulting from the effect of a D. Obiassing potential, a variable acoustical impedance will result. Hence, undesired deviations from the desired phase displacement will be experienced resulting in an arrangement exhibiting frequency sensitive directional properties. Another disadvantage of the microphone arrangement described above is that it is difficult to disassemble.

With a viewto overcoming the above disadvantages it is an object of the present invention to provide a micro- .phone arrangement which is easily constructed and which vide a microphone arrangement having a flat frequency response over a broad frequency band.

Still another object of the present invention is to provide a microphone arrangement which is very compact in structure.

With the above objects in view, the present invention "mainly co'nsists'of a microphone arrangement comprising a pot-shaped hou'singhaving a rear'wall, a side wall. andbeingopen at one end'sothat the sidewall has afree edge, the housing being provided'with 'a plurality of radiallyspaced apertures "passing through the wall thereof,

a substantially fiat'membrane closing the open end of the housing and secured along its edgeto the edge of "the side'wa1l of the housing so as to close the same, a

perforated rear electrode arranged in the housingisu'pported by the same and spaced with its front face at a slight distance frorn'the substantially flat membrane so 'asto'form'a gap between itself and the1membrane,the "gap communicating with the apertures passingthrough "the"wall ofthe housing, the'rear electrode being also the atmosphere surrounding t .e housing.

Thenovel features which are considered as characteristic forthe invention are'set forth inparticular in the appended claims. The invention itself, however, both ".as to its 'construc'ti'on'and itsmethcd of operation, to-

gether with additional objects and advantages thereof,

7 will'be best understood "from the toll swing description 1of'specific"embodimentswhen read .in connection with the accompanying drawings, in which:

Fig. lisa'vertical sectiono'f a'microphone arrangement in accordance'with the invention taken" throughlmdlI- II Fig. 2 is a plan view taken in section through line I- wardly approaching sound waves to strike the front face of Fig. l in direction of the arrows; and

Fig. 3 is a vertical section of another embodiment of a microphone arrangement in accordance with the invention.

Referring to Figs. 1 and 2, a substantially fiat membrane, or diaphragm 1, is adapted to be vibrated by acoustical oscillations. A substantially plate-shaped rear electrode 4 is arranged parallel to and slightly spaced from one face of the membrane 1. The rear electrode 4 is formed with a plurality of perforations 12, shown more clearly in Fig. 2, all of which are spaced from the edge of the plateshaped rear electrode 4 so as to form along the edge of said rear electrode 4 an annular perforation-free gap 8 between itself and the membrane 1. The annular perforation-free gap 8 acts as an acoustical resistance. A membrane housing 2 is covered at its end opposite the membrane 1 by an apertured insulating plate 7 which serves not only to support the rear electrode spaced from the plate 7, as well as from the housing 2, and the membrane 1, but also serves to effectively isolate the rear electrode from the housing, or membrane, which is electrically connected with the housing, so that an electrical output may be obtained between the rear electrode and the membrane, or housing, which is proportional to the acoustical oscillations striking the membrane 1. As may be noted in Fig. l, a space is formed between the rear electrode 4 and its supporting plate '7 forming an air chamber 5 which serves as an acoustical capacitance.

A plurality of radially spaced openings 6' pass through the housing wall and serve for permitting the passage of sound waves from the atmosphere surrounding the housing into the gap 3 formed between the housing wall and the rear electrode 4. Arranged about the housing in the region of the plurality of openings 6', is an adjusting ring 9 provided with a plurality of radially spaced openings 6 being equal in number and in spacing to the number and spacing of the plurality of radially spaced openings 6 formed in the housing 2. The ring 9 serves to selectively open or block the plurality of radially spaced openings 6' so as to prevent the same from communicating with the atmosphere surrounding the housing 2. When the holes 6 are not blocked, a microphone having a one sided directional pattern is obtained. If the holes 6' are blocked, by the positioning ring 9, a microphone having a nondirectional pattern will be obtained. A cover 10 serves to secure the diaphragm 1 to the housing 2.

The openings 6 and 6 formed in the adjusting ring 9 and the housing 2, respectively, should be large enough to present practically no resistance to acoustical oscillations entering these openings. The same applies for the gap 3. a

In order to obtain a microphone exhibiting the fiat frequency response over a broad frequency band the acoustical resistance should be capacitance in specific manner. The flat frequency response of the microphone depends on the design of the phase displacing member, which as already noted, is made up of an acoustical resistance and an acoustical capacitance which are in turn respectively formed by the annular perforation-free gap 8, and the air chamber 5.

In order to obtain a microphone exhibiting a one-sided directional pattern, as already mentioned, the adjusting ring 9 is adjusted so that the plurality of radially spaced openings 6, formed therein, communicate with the radially spaced openings 6' formed in the housing 2 thereby permitting sound waves approaching the microphone to enter the same through the openings 6 and 6. Since in most cases, it is preferred to intercept sound waves approaching the microphone from a frontward direction, it is necessary in order to make the microphone sensitive only to sound approaching in that direction to use a phase displacing member which will delay the sound waves, approaching the microphone from a direction behind the same, for a time equal to that which it takes for the rearrelated to the acoustical of the membrane'of the microphone.

The sound waves approaching from the rear of the microphone are delayed a time 1 before striking the front membrane of the microphone. The delay time, t, is equal t0 where a is the average sound path deviation, that is, the distance sound travels from a point on one face of the membrane to a corresponding point on the other face thereof, and v is the sound velocity.

In order to obtain a time delay of the sound waves which is independent of frequency the value of r, the acoustical resistance, must be small in relation to the value of where w=21rf, and where C represents the acoustical capacitance. With the above relationship of r and C the phase difference go between the sound pressure behind the annular perforation-free gap 8, which acts as an acoustical resistance, and the sound pressure on the rear side of the membrane is given by (p='y.w.C. Hence, the phase delay time It is to be noted that the phase delay time I; produced by the phase displacing member is not a function of frequency. The phase displacement obtained is in dependent of frequency over a frequency range of several thousand cycles. Beyond a given frequency point r becomes mueh greater than and as a result the annular perforation-free gap 8 will effective isolate the air chamber 5 from the external atmosphere surrounding the housing 2. At this frequency range the microphone is converted into a pressure microphone exhibiting a non-directional characteristic which may be modified, however, to give the desired one-sided directional characteristic by flexing the microphone body.

With the arrangement in accordance with the invention, the acoustical resistance remains constant over the entire frequency band. The magnitude of the acoustical resistance depends upon the distance between the rear electrode 4 and the membrane 1 as well as the width of the annular perforation-free gap 8. Furthermore, in the arrangement in accordance with the invention the inner arranged series of holes provided in the rear electrode have practically no influence on the acoustic resistance of the annular perforation-free gap 8. The air chamber 5 serves as an acoustic capacitance, as heretofore noted, and produces the desired phase displacement in conjunction with the acoustic resistance.

When the sound waves are permitted to enter the microphone through the openings 6 and 6, they reach the air chamber 5 by passing through the low acoustical resistance gap 3, the annular perforation-free gap 8, and almost exclusively through the outer series of holes provided in the rear electrode 4. From the air chamber 5 the delayed sound waves pass through the inner series of holes provided in the control electrode 4 and impinge on the microphone membrane 1 so as to excite the same. The propagation of thesound waves past the annular perforation-free gap 8 has practically no influence on the membrane since the gap 8 is arranged at the periphery of the diaphragm and the sound waves are supplied with a low resistant path through the outer openings formed in the rear electrode.

In contradistinction to the prior art arrangement wherein the acoustic resistance and capacitance are distributed over the entire surface of the rear electrode in such manner as to interact with one another, the arrangement in accordance with the present invention presents an acoustical resistance and an acoustical capacitance which are distinctly separate from one another. Moreover, the effect of the acoustical resistance is concentrated over a limited defined area thus overcoming the problem of diaphragm membrane sag. The acoustic capacitance is also seen to be concentrated within a defined area, namely the air chamber 5.

Fig. 3 illustrates a microphone arrangement which is basically the same as that illustrated in Figs. 1 and 2 with the exception that the plurality of openings 11 communicating with the outside atmosphere are provided in longitudinal direction of the housing rather than in lateral direction thereof as illustrated in Fig. 1. In this arrangement the housing 2' is provided with a threaded end portion permitting an adjusting ring 9' provided with a screw portion to be screwed thereto. With the adjusting ring 9' screwed in the position shown in Fig. 3 the air chamber 5 is free to communicate with the external atmosphere surrounding the housing. If it is desired to block the openings 11 provided in the insulating plate 7', the adjusting ring 9' may be screwed in clockwise direction until it touches the bottom surface portion of the insulating plate 7'. With this arrangement no lateral openings in the wall of the housing 2 are necessary. All other structural details set forth with respect to the structure of Figs. 1 and 2 apply to the structure illustrated in Fig. 3.

A particular advantage of the microphone arrangement in accordance with the invention is that it may be easily disassembled. Another advantage of a microphone arrangement in accordance with the invention is that the isolating member normally provided on the top surface of the rear electrode to isolate the same from the membrane, may be eliminated.

Finally because of the small and advantageous dimensions of the microphone arrangement in accordance with the invention, and because of the manner in which the openings are provided in its housing, the microphone may be directly connected across the input terminals of an amplifier.

While the invention has been illustrated and described A as embodied in electro-acoustical transducers, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

l. A microphone arrangement comprising, in combination, a substantially fiat membrane adapted to be vibrated by acoustical oscillations; a substantially plateshaped member arranged parallel to and slightly spaced from one face of said membrane and having a plurality of perforations all of which are spaced from the edge of said plate-shaped member so as to form along the edge of said plate-shaped member an annular perforation-free gap between said plate-shaped member and said membrane; and a pot-shaped housing having a rear wall and side wall and secured to said membrane along the free edge of said side wall supporting said membrane, said housing also supporting said plate-shaped member within the housing spaced from the rear wall thereof so as to form between said plate-shaped member and said rear wall a closed air chamber communicating through said perforations with said annular gap, said housing also having at least one small aperture passing through the housing wall and communicating on the one hand with said annular gap between said membrane and said plateshaped member and on the other hand with the atmosphere surrounding the housing.

2. A microphone arrangement comprising, in combination, a tubular housing formed with a plurality of openings; a substantially fiat membrane adapted to be vibrated by acoustical oscillations and arranged on one end of said housing; a tubular member having an apertured plate portion at one end thereof and being closed at the other end thereof to form a closed chamber; and support means connected to the other ends of said housing and said tubular member for supporting the latter radially spaced from said housing, so as to form between said tubular member and said housing a first gap communicating with said plurality of openings, and with said apertured plate portion arranged adjacent to and spaced from said membrane so as to form a second gap extending between the adjacent surfaces of said membrane and said apertured plate, said second gap having a peripheral gap portion extending between the peripheral edge of said apertured plate and the outermost openings therein to form an acoustical resistance, said peripheral gap portion communicating with the external atmosphere by means of said first gap communicating with said plurality of openings in said housing and communicating with said chamber through said apertured plate.

3. A microphone arrangement comprising, in combination, a substantially flat membrane adapted to be vibrated by acoustical oscillations; a substantially plate-shaped member arranged parallel to and slightly spaced from one face of said membrane and having a plurality of perforations all of which are spaced from the edge of said plate-shaped member so as to form along the edge of said plate-shaped member an annular perforation-free gap between said plate-shaped member and said membrane; and a pot-shaped housing having a rear wall and side and secured to said membrane along the free edge of said side wall supporting said membrane, said housing also supporting said plate-shaped member within the housing spaced from the rear wall thereof so as to form between said plate-shaped member and said rear wall a closed air chamber communicating through said perforations with said annular gap, said housing also having a plurality of radially spaced apertures passing through the housing wall and communicating on the one hand with said annular gap between said membrane and said plate-shaped member rear and on the other hand with the atmosphere surrounding the housing.

4. A microphone arrangement comprising, in combination, a substantially flat membrane adapted to be vibrated by acoustical oscillations; a substantially plateshaped member arranged parallel to and slightly spaced from one face of said membrane and having a plurality of perforations all of which are spaced from the edge of said plate-shaped member so as to form along the edge of said plate-shaped member an annular perforation-free gap between said plate-shaped member and said membrane; and a pot-shaped housing having a rear wall and side wall and secured to said membrane along the free edge of said side wall supporting said membrane, said housing also supporting said plate-shaped member within the housing spaced from the rear wall thereof so as to form between said plate-shaped member and said rear wall a closed air chamber communicating through said perforations with said annular gap, said housing also having a plurality of longitudinally extending apertures passing through the rear wall of the housing and communicating on the one hand with said annular gap between said membrane and said plate-shaped member and on the other hand with the atmosphere surrounding the housing.

5. A microphone arrangement comprising, in combination, a tubular housing formed with a plurality of openanswer ings; a substantially fiat membrane adapted to be vibrated by acoustical oscillations and arranged on one end of said housing; a tubular member having an apertured plate portion at one end thereof and being closed at the other end thereof to form a closed chamber; support means connected to the other ends of said housing and said tubular member for supporting the latter radially spaced from said housing, so as to form between said tubular member and said housing a first gap communieating wtih said plurality of openings, and with said apertured plate portion arranged adjacent to and spaced from said membrane so as to form a second gap extending between the adjacent surfaces of said membrane and said apertured plate, said second gap having a peripheral gap portion extending between the peripheral edge of said apertured plate and the outermost openings therein to form an acoustical resistance, said peripheral gap portion communicating with the external atmosphere by means of said first gap communicating with said plurality of openings in said housing and communicating with said chamber through said apertured plate; and means operatively connected to said housing in the region of said plurality of openings for selectively opening and blocking said plurality of openings so as to prevent the same from communicating with the atmosphere surrounding said housing.

6. A microphone arrangement comprising, in combination, a housing formed with openings passing therethrough and being open at one end thereof; a membrane adapted to be vibrated by acoustical oscillations secured to said housing and closing said open end of the same; and an inner member arranged and supported in said housing and defining with the same a passage communicating with said opcning$, said inner member including a wall arranged parallel to and spaced from said membrane, said wall having a central portion formed with radially spaced sets of perforations and an annular edge portion defining with an annular portion of said membrane a narrow annular gap constituting an acoustical resistance and communicating with said openings in said housing through said passage, said inner member being formed with a closed air chamber constituting an acoustical capacitance and communicating with said perforations in said central portion of said wall so that acoustic oscillations pass through said openings in said housing, said annular passage, said annular gap, and through said perforations into and out of said air chamber to oscillate said membrane.

7. A microphone arrangement as claimed in claim 6 wherein said housing is pot-shaped and includes a tubular side wall in which said openings are formed, and wherein said inner member includes a tubular wall radially spaced from said tubular side wall to define said passage so that said passage is annular.

References Cited in the file of this patent UNITED STATES PATENTS 2,387,845 Harry Oct. 30, 1945 

